1. Field of the Invention
The invention is generally related to fuel assemblies for nuclear reactors and more particularly to the guidance of fuel rods into fuel assemblies.
2. General Background
Fuel assemblies for nuclear reactors are formed from a number of fuel rods held in position by spacer grids located along the length of the fuel rods. The spacer grids, crisscrossing metal strips on edge, provide an individual cell for each fuel rod. Some spacer grids are provided with a series of projections, grid mixing vanes, which serve to cause a predetermined turbulence and mixing of coolant during flow through the spacer grids and around the fuel rods.
The remote guidance of fuel rods into the spacer grids of the fuel assembly presents difficulties which can result in damage to the fuel rods and the spacer grids. During insertion the fuel rods can snake or mis-track into an adjacent grid cell which could cause unrepairable damage to the fuel rod and the spacer grids. The bending of grid mixing vanes is also a problem which can occur without rod snaking. Such a problem is difficult to detect and results in reduced performance of the assembly. The most common method used in the nuclear industry when remotely loading a fuel assembly is to load the new fuel assembly skeleton with non-fuel bearing rods (dummy rods) in every other cell. This creates a checkerboard array around the empty cells where fuel rods are to be inserted. After insertion of the fuel rods into the empty cells the dummy rods are removed one at a time to provide empty cells for insertion of the remaining fuel rods needed to complete the fuel assembly. Although this method works well in preventing fuel rods from snaking into adjacent cells it does not prevent the rods from bending grid mixing vanes. Also, the handling of the dummy rods significantly increases the time required to complete rod transfers. Storage of the dummy rods is also a problem after they have been introduced into the contaminated water of a site's spent fuel pool. This waste problem must then be addressed either by the provision of additional storage space or by decontamination of the dummy rods, both of which increase costs. Because of cell relaxation due to the dummy rods being inserted into the cells, special consideration must also be given to the sizing of grid cells when manufacturing a new skeleton. Resizing the cells accordingly can also increase the cost associated with manufacturing and licensing of the new skeleton.
Devices related to fuel assemblies which applicant is aware of include the following.
U.S. Pat. No. 4,317,702 discloses the use of rack elements having square fuel assembly cluster-receiving tubes which are fastened to bed plates at a small distance for storing fuel assembly clusters in a water pit.
U.S. Pat. No. 4,358,421 discloses the use of a nuclear fuel assembly lock structure for control rod guide tubes to allow removal and replacement thereof.
U.S. Pat. No. 4,448,744 discloses a method of loading and unloading a nuclear reactor wherein at least one of four fuel assemblies in a group is unloaded, a support member is inserted in place of the withdrawn fuel assembly, and then withdrawing the control rod associated with the four fuel assemblies.
U.S. Pat. No. 4,673,544 discloses the use of a plurality of axially shiftable rods disposed for pushing from a nuclear reactor fuel assembly fuel rods in the same geometric pattern and pitch as the shiftable rods.
U.S. Pat. No. 4,676,945 discloses the use of a centering device of a generally box-like cap configuration that engages the upper tie plate of a fuel bundle. Tapered dowel pins engage apertures in the tie plate for transmitting lateral forces to the fuel bundle assembly. An elongated handling pole is attached to the centering device by a ball and socket arrangement.
U.S. Pat. No. 4,788,028 discloses the use of a horizontally movable platform having a vertical telescopic mast with a gripper for gripping the upper end piece of a fuel assembly.
U.S. Pat. No. 4,793,962 discloses the use of a cassette having recesses for receiving fuel rods therein from a nuclear fuel assembly. When the cassette is full simultaneous pressure on all the rods thrusts them into a storage case.
As seen from the above, the generally used method of inserting dummy fuel rods leaves room for improvement and the known patents do not address the problem of properly inserting fuel rods into spacer grids of fuel assemblies.